US crude oil down %2 right now, $51 a barrel after inching up in the beginning of that year. The prices have been falling for a month now as the deal gets closer.
Iran has the 4th biggest oil reserve in the world. That oil has been blocked from the oil market by sanctions. This deal would change that and allows Iran to increase its oil exports. This could take months, may take year for Iran to fully ramp up its production. But eventually a deal could add one million barrels of crude oil per day to the gobal market. That would be added to the already huge surplus, almost two million barrels a day above demand.
Adding more oil to the market would push prices lower. Oil could soon be brought back in to the $40 range, which means gas prices could return to $2 per gallon.